1. Field of the Invention
The present invention relates to a start-up circuit for a direct current (DC) fan, and more particularly to a start-up circuit which has a decreased start-up current and which stabilizes running of a DC fan.
2. General Background
Developments in today's highly information-intensive society have led to remarkable improvements in performances of electronic devices. During operation of many contemporary electronic devices such as central processing units (CPUs), large amounts of heat are produced. Typically, an electric fan is used to facilitate removal of the heat. The fan must run stably, so as to prevent the device from becoming unstable or being damaged. A start-up circuit accompanying the fan is quite important to ensure normal running of the fan.
FIG. 5 shows a conventional start-up circuit of a DC fan. A Pulse-Width Modulation (PWM) signal from a control chip directly drives transistors Q70 and Q100. The transistors Q70 and Q100 directly drive a DC fan. A high capacity electrolytic capacitor C63 is provided for wave filtering. Referring to FIG. 6, when the start-up circuit is started, a large start-up current is generated at that instant. The start-up current is liable to damage or even ruin the transistors Q70 and Q100. In addition, although the high capacity electrolytic capacitor C63 is employed, voltage ripples may still occur. When this happens, the fan may rotate unstably.
FIG. 7 represents a start-up voltage control circuit of a DC brushless fan, as disclosed in Taiwan Patent Application No. 092215559. The control circuit includes a comparator 1, a voltage sampling circuit 2, and an on-off control circuit 3. The comparator 1 compares a supply voltage of a supply circuit with a reference voltage that meets with a fan driving voltage specification. The on-off control circuit 3 is connected to an output terminal of the comparator 1, for controlling whether a drive circuit 4 and a fan motor 6 are connected to the supply circuit. The on-off control circuit 3 comprises transistors Q1 and Q2. However, the transistors Q1 and Q2 are directly controlled by the supply circuit. When a starting current passing through the transistors Q1 and Q2 is too large, the transistors Q1 and Q2 are liable to be damaged or even ruined.
What is needed is a start-up circuit which has a decreased start-up current and which can stably run a DC fan.